The present invention relates to a toy driven by a small capacity motor. More particularly, the invention relates to an automatic variable torque transmission device for model cars or vehicles.
There have been previously proposed numerous types of transmission mechanisms for motor driven toys. However, there has been a strong demand for miniaturization and simplification of the transmission mechanism due to the fact that the motor used in such toys has a small capacity.
One type of such a transmission for motor driven toys is disclosed in U.S. Pat. No. 3,540,152. The transmission mechanism disclosed in that patent is relatively simple and may be miniaturized. However, the transmission has inherent defects due to its transmission clutch construction. That is, at a high speed and low torque, it is impossible to obtain smooth rotational power transmission and, at a low speed and high torque, the rotational torque must be transmitted to the output shaft through a clutch. Also, since the transmission clutch must be of a one-way type, it is impossible to reverse the vehicle using a motor generated torque.
More specifically, the transmission mechanism described in the reference patent will be explained with reference to FIGS. 1 to 3. FIG. 1 is a partial cross-sectional plan view showing a primary part of the mechanism in a normal high-speed low torque state and FIG. 2 is a similar plan view showing a low speed high-torque state thereof. A small capacity motor, torque reducing means, frame body and other associated parts are omitted in the drawings. In FIG. 1, reference numeral 10 denotes a gear engaged with an output gear of a torque reduction mechanism (not shown). A high speed gear 12 having a large diameter and a slow speed pinion 14 having a small diameter are mounted to always rotated with the gear 10 on a shaft indicated by a dotted line 16. During gear rotation, the gears 10, 12 and 14 are not displaced in the axial direction. On an output shaft 18 is fixedly disposed driven means such as a pair of tires 20, a stopper sleeve 22 and a cam sleeve element 24. An associated cam sleeve portion 26 is engaged with the fixed cam sleeve element 24 through slant cam surfaces 25 and 27. The portion 26 is formed integrally with an output gear 28 which engages with the high speed gear 12 in the high speed state shown in FIG. 1 and which is slidable in the axial direction. A high load gear 30 is mounted to freely rotate on the shaft 18 and is always engaged with the slow speed pinion 14 and biased toward the output gear 28 by a spring 36. A clutch, generally denoted by reference numeral 38, is of one-way type.
FIG. 3 is a perspective view illustrating the contruction of the clutch mechanism 38. The high load gear 30 has on its inner side a pair of projections 40 each of which includes an abrupt surface 32 and a gradually rising surface 34 as shown in FIG. 3. The output gear 28 is also provided with a pair of oppositely arranged similar projections 42 each engageable with the projections 40 of the high load gear 30. The one-way riding-over clutch 38 is thus constructed.
The transmission device operates as follows. For normal high speed operation, the output gear 28 and cam sleeve portion 26 are rotated in the direction indicated by the arrow 44 by a high speed gear 12 with torque being transmitted from the motor through the reduction gears and gear 10. At this time, the output gear 28 and the cam sleeve 26 tend to move axially leftward due to the provision of the cam engagement but the spring 36 serves to balance or engage the cam sleeve 28 and the fixed cam sleeve 24 through the clutch 38. Thus, while maintaining meshed engagement between the high speed gear 12 and the output gear 28, the rotational torque is transmitted to the tires 20. However, it should be noted that during normal high-speed low-torque operation of this transmission system, there is a difference in circumferential speeds between the high speed gear 12 and the slow speed pinion 14. The low speed pinion 14 is always engaged with the high gear 30 which is freely rotatable around the output shaft 18. The circumferential speed difference is absorbed by the one-way clutch 38 where the projections formed in the inner surface of the output gear 28 ride over the associated projections 40 formed in the high-load slow gear 30 which results in poor durability and rotational speed fluctuations.
Moreover, a vehicle provided with such a transmission device cannot be operated in reverse. That is, since the output gear 28 having the smaller number of gear teeth and the high torque gear 30 having the greater number of gear teeth, which are both provided on the same shaft 18, are rotated according to the outputted torque transmitted by the two gears 12 and 14 having different number of gear tooth and with the gears 12 and 14 provided on the same shaft 16, only one-directional rotation of the output shaft 18 is possible using the riding-over action of the one-way clutch 38. In a vehicle provided with such transmission system, if the reverse rotational torque were to be transmitted to the gears 12 and 14, the motor would stall because the one-way clutch projections 40 and 42 are engaged with each other.
For low-speed high torque operation, the transmission system operates as follows. When the vehicle runs on a steep grade or with a high load otherwise put on the tires, the cam sleeve 26 and the output gear 28 are pushed toward the left against the force of the spring 36 so that the output gear 28 disengages from the high speed gear 12. Then, the high load gear 30 is pushed leftward through the clutch 38 until its end abuts the stop 22. The gear 30 is thus engaged only with the slow speed gear 14 as shown in FIG. 2. The torque transmitted to the high load gear 30 is remarkably increased and the increased torque is further transmitted to the output shaft 28 through the clutch in which the projections 40 and 42 are fixedly engaged by the lateral component of force generated due to the tire load. Thus, the torque must be transmitted to the output shaft 18 through a somewhat intricate clutch construction during high load operation. This is an inherent defect to the prior art device.